Bis(alkyny1) complexes Ru(CWR12P4 (1-3) (R = Ph,p-tolyl, tBu; P = P(OMe)3 (11, P(OEt)3 (2), PPh(0Et)z (3)) were prepared by reacting RuCl2P4 with excess Lif RC=C-, and a trans geometry was established both in solids (X-ray) and in solution. The reaction of these alkynyls (1 -3) with electrophilic reagent depends on the nature of the phosphite ligand. Vinylidene-acetylide derivatives [RU(CGCR){=C=C(R~)R}P~I+ (R1 = H (4, 51, CH3 (7, 8), ArN-N (lo), I (12), 2,3-(No~)&&S (14)) were prepared with P(OMe)3 and P(OEt)3 ligand by treatment of 1 and 2 with HBF4, CF3S03Me, ArNz+BF4-, Iz, and ~,~-( N O Z ) Z C~H~S C~, respectively. Instead, only the diazo-and iodovinylidenes [Ru(CsCR){ =C=C(R1)R)P41f (RI = p-tolN-N (ll), I (13)) were obtained with the PPh(0Et)z phosphite ligand. These vinylidene compounds were fully characterized by IR, IH, 31P, and 13C NMR spectra, and a single-crystal X-ray structure determination of complex [Ru(C=CPh){=C=C(Me)Ph}-{P(OEt)3}4]CF3S03 (8a) is reported. The alkynyl-vinylidene [Ru(C=CR){=C=C(H)R}P4]+ cations (4, 5) rearrange in solution to enynyl [Ru(q3-RC3CHR)P41f derivatives, and the reaction is inhibited by the presence of free alkyne. Kinetic data support a mechanism involving a pentacoordinate intermediate formed by loss of the vinylidene ligand. Substitution of the =C=C(H)R ligand by phosphite, isocyanide, and nitrile is easy in 4 and 5 and leads to [Ru(C~CPh){P(OMe~3}P41+ -(17), tolCN)2P4l2+ (19) (P = P(OEt)3), derivatives. ligand in 4 and 5, giving Ru(C=CR)zP4, was Introduction A large number of studies on the chemistry of transi-